1. Field of the Invention
The present invention relates to a servo device for an optical actuator in an optical disc player, more particularly to a focus servo circuit for an optical pickup actuator in an optical disc player.
2. Description of the Prior Art
In general, an optical disc player for playing back an optical disc such as a mini disc or a compact disc employs an optical pickup device for exactly reading information recorded on the optical disc. Such an optical pickup device has an optical pickup actuator equipped with an objective lens. The optical pickup actuator is controlled by a servo device therefor. According to a control of the servo device, the optical pickup actuator performs a focusing operation and a tracking operation. The objective lens of the optical pickup actuator moves up and down by the focusing operation of the optical pickup actuator.
FIG. 1 is a schematic view for explaining a flow of a laser beam from a generation of the laser beam to outputs of currents for controlling tracking and focusing operations of a servo device. As shown in FIG. 1, a laser beam generated from a laser diode 150 reaches a half mirror 154 through a grating lens 152. The laser beam which has reached half mirror 154 is irradiated on an optical disc 156 through an objective lens 66 of an optical actuator. The laser beam irradiated on optical disc 156 is reflected to half mirror 154 through objective lens 66 again. A part of the laser beam reflected from half mirror 154 is irradiated on a photo diode 10.
At this time, photo diode 10 outputs an electrical signal I.sub.P according to the laser beam irradiated thereon. Electrical signal I.sub.P is inputted to a servo device 160 for an optical pickup actuator. Servo device 160 outputs tracking and focusing control currents I.sub.t and I.sub.f according to electrical signal I.sub.P inputted thereto. Currents I.sub.t and I.sub.f are applied to a tracking coil and focusing coil (not shown) of the optical actuator, so that the tracking and focusing operations are controlled by currents I.sub.t and I.sub.f.
FIG. 2 is a block diagram for illustrating a conventional focus servo circuit for an optical pickup actuator. As shown in FIG. 2, the laser beam reaches photo diode 10 through optical disc 156 and objective lens 66. Photo diode 10 is divided into 4 regions A, B, C, and D. When electric power is applied to the optical disc player, a focus servo circuit 163 of an optical pickup actuator 8 outputs a focus servo start signal with a microprocessor 5 operating. The focus servo start signal is applied to a focus servo drive unit 7 of optical pickup actuator 8 by a control of a switching unit 6 through a switching control unit 165. Therefore, optical pickup actuator 8 performs the focusing operation of moving objective lens 66 up and down. Such up and down movements of objective lens 66 are detected by photo diode 10 thereby outputting a focus error signal FE. Focus error signal FE is inputted to switching control unit 165 which detects a zero crossing of focus error signal FE. Since objective lens 66 can be linearly controlled in a predetermined period section of focus error signal FE when the zero crossing is detected, switching control unit 165 cuts off outputting the focus servo start signal and enables focus error signal FE from photo diode 10 to be applied to a focus error detector 11. Therefore, focus servo circuit 163 performs a normal focusing operation for optical pickup actuator 8 following the detection of the zero crossing of focus error signal FE.
As mentioned above, in a conventional focus servo circuit for an optical pickup actuator, since a normal focusing operation is started after a detection of a zero crossing, there exit drawbacks in that a normal focusing operation is delayed since a normal focusing operation is started after a detection of a zero crossing and in that a normal focusing operation may be started by a focus error signal caused by disturbance abruptly occurring from the outside.